Search

Hydrogenated diamond‐like carbon (H‐DLC) films are limited by their poor thermal stability, which significantly affects the tribological applications and needs improvement. Accordingly, nanodiamond (ND) and hexagonal boron nitride (h‐BN) are used to address this issue. When the H‐DLC surface is deposited using ND+h‐BN mixture with mass ratio of 1:1 and a concentration of 0.1 mg mL−1 and vacuum‐heated at 200 °C and 1 × 10−5 Pa, a superlow friction coefficient of 0.0015 can be obtained, with a reduction of 98.33% as compared to pure H‐DLC. Correspondingly, the wear rates of wear scar and wear track decreased by 81.95% and 24.83%, respectively. High vacuum thermal treatment can purify the adsorbed species on the surfaces of ND and h‐BN, and produce newly‐exposed dangling bonds. Simultaneously, new bonds of C‐N are formed between ND and h‐BN, and the nanoparticles adhere together to form a polymer‐like structure under friction. Furthermore, the ND can support h‐BN and reduce its agglomeration. Under the action of tribochemical reaction, the layer spacing of hexagonal boron nitride is increased to obtain a better shear slip. The combination of these factors resulted in ultra‐low friction. This study paved the way for developing functional anti‐friction additives for durable and high‐performance lubrication. [ABSTRACT FROM AUTHOR]

The formation of tribolayers may play significant influences on fretting wear. At elevated temperature, the adhesion among wear debris and the increased diffusion rate facilitate the formation of tribolayers. However, the intensification of oxidation at elevated temperature and the low diffusion rate in oxides may play an adverse role. The present study aims to investigate the role of temperature in tribolayers in fretting wear using a γ-TiAl alloy. Scanning electron microscope, energy dispersive spectrometer, Raman spectrometer, transmission electron microscope and nanoindentation were utilized to investigate the wear debris, tribolayers, and wear scars. The fretting tests showed that, compared with that at room temperature (RT) and 350 °C, significant reduction in wear rate and decrease in the fluctuation of friction coefficient occurred at 550 and 750 °C. It was further revealed that when temperature raised from room temperature (RT) to 750 °C, the oxidation of the wear debris increased slightly and the diffusion coefficients increased prominently, which facilities the formation of well tribo-sintered tribolayers. The well tribo-sintered tribolayers presented homogenous structure, nanocrystalline grains with excellent mechanical properties, and resulted in the improvement in the fretting wear resistance of the γ-TiAl alloy at 550 and 750 °C. [ABSTRACT FROM AUTHOR]

The tribological properties of γ-TiAl alloys under sliding condition have been well established, but the mechanisms for fretting wear, especially at elevated temperature, remain to be further clarified. In the present study, the tribological behaviors and mechanisms of a γ-TiAl alloy were investigated under an elevated temperature fretting condition. The wear scars, debris, and tribolayers were studied with scanning electron microscope, energy dispersive spectrometer, and Raman spectrometer. The structural and mechanical properties of the tribolayers were investigated utilizing transmission electron microscope and nanoindentation tests. The results demonstrated that the severe adhesive wear, delamination, and abrasive wear in the early stage of fretting resulted in high wear rate of the γ-TiAl alloy. As fretting progresses, the formation of tribolayers with nanograins and excellent mechanical properties improved the fretting wear resistance of the γ-TiAl alloy. Due to the protective effect of the adhesively transferred γ-TiAl, the wear rate of the nickel-based superalloy counterpart was significantly lower. [ABSTRACT FROM AUTHOR]

Wear behavior of three different AA6061-0.5 wt% CNT composites fabricated using different dispersion techniques by stir-squeeze-casting have been studied against a steel ball counter body. The uniform CNT distribution in composites fabricated by dropping pellets of Al-powder and CNT (PD composite) yields the lowest coefficient of friction (COF) and specific wear rate (SWR) for both the composite material and the counter body. Both COF and SWR increase as the load increases from 5 N to 10 N, but drop as the load increases further to 15 N. Even at 150 °C, the presence of CNTs improved the tribological performance of AA6061. A detailed study of CNT morphologies on the worn surface and Raman spectroscopy indicate their beneficial role as a solid lubricant to reduce COF and material loss during wear. • Wear behavior of Al-CNT composite strongly affected by CNT dispersion in Al melt. • The COF and SWR of pellet drop composites (PD) were 68 % and 77 % lower than AA6061. • At 150 °C, PD composite is more wear resistance than AA6061 even at higher load. • Stability of tribolayer determines the wear behavior of the composites. • Raman Peak shift and I D /I G ratio indicate structural change of CNTs due to wear. [ABSTRACT FROM AUTHOR]

Doping with non-metal element is effective to improve the porous morphology and enhance the mechanical and tribological properties of transition metal dichalcogenides (TMDs) based film. However, the mechanical properties of TMDs based film with single additive should be enhanced further for better wear resistance. In this work, B/N dual-doping was used to further enhance the deformation resistance and toughness of MoS 2 based films deposited by magnetron sputtering, while the effect of B/N dopants on the structure, mechanical and tribological properties has been investigated. Mo-S-B-N film with B content of 20.91 at. % and N content of 18.13 at. % consists of MoS 2 and amorphous nitride/boride, in which B/N dual doping film forms a higher ordered MoS 2 lamellar structure compared to B doping film. With a hardness of 11.6 ± 0.9 GPa and improved toughness, the film achieves a low wear rate and average friction coefficient is achieved. Comparing B/N dual doping film to B doping film, the stable formation of tribolayer with more strengthened deformation resistance results in the steady friction and the improved wear resistance, while the limited TMDs reorientation leads to the slight increase of friction coefficient. • B/N dual-doping was proposed to prepare the reinforced MoS 2 based sputtered films. • The amorphization of MoS 2 attributed to excessive B dopants were suppressed by N addition. • Wear mechanism of B/N dual-doping MoS 2 based films are investigated by AFM and chemical component analysis on wear track. [ABSTRACT FROM AUTHOR]

Improving the tribological properties of bearing steels in starved lubrication is of practical significance. In this study, niobium and nitrogen (Nb+N) implantation were utilized to enhance the performance of 8Cr4Mo4V steel. Sliding wear test, contact angle measurement, nanoindentation, X-ray photoelectron spectroscopy, and transmission electron microscope examination were conducted. Nb+N implantation results in friction reduction and a remarkable (84.5 %) decrease in wear rate. Experimental analysis reveals that Nb+N implantation enhances surface hardness and improves oil retention capacity which benefits wear resistance. Moreover, the tribolayers for implanted 8Cr4Mo4V demonstrate nanocrystalline structure with fewer defects, further contributing to the wear resistance. The findings provide new insights into the tribological mechanisms and highlight the potential of ion implantation in starved lubrication scenarios in aerospace. [Display omitted] • Nb+N implantation improved mechanical properties and oil retention of 8Cr4Mo4V. • Nb+N implantation increased subsurface dislocation density. • Nb+N implantation reduced crack generation and breakdown of tribolayers. • Reduced friction and improved wear resistance were achieved by Nb+N implantation. [ABSTRACT FROM AUTHOR]

This research aims to fabricate Cu-10Sn-5Ni/10wt%TiC functionally graded composite (Øout100 X Øin70 X 100 mm) through horizontal centrifuge cast technique and to study its tribo-mechanical characteristics. Microstructural observation revealed higher percentile of reinforcement particles at inner wall zone compared to middle and outer zones. Vicker's micro hardness tests revealed proportional increase of hardness along the radial wall towards inner periphery. Inner (7-15 mm) zone had higher tensile strength (291 MPa) compared to outer (1-7 mm) zone (236 MPa). Non-lubricated tribology experiments were performed at inner, middle and outer region of composite using pin-on-disc tribometer under a wide range of loads, sliding distances and sliding velocities. From the wear experiment results, it was observed that inner region had minimal wear due to rich particle presence. Scanning Electron Microscopy on worn specimen at low and high wear parametric conditions exhibited features like delamination, ploughing, micro cutting, particle pull-out, plastic deformation with flaky wear debris and thin film formation. [ABSTRACT FROM AUTHOR]

Abstract Aluminum-graphene nanosheets (GNSs) composite was fabricated and subjected to dry sliding wear tests. A protective and lubricious tribolayer was formed between the sliding interfaces. SEM, TEM and Raman microscopies were used to characterize the cross section and top surface of the tribolayer, for the purpose of understanding the effect of GNSs on the microstructure of the tribolayer and induced tribological performance. It was found that the incorporation of 1 vol% GNSs reduced the wear and friction by 85.0% and 39.1% respectively. GNSs strengthen the formation of the protective tribolayer by wrapping sliding surface debris particles and bridging subsurface cracks. Raman mapping results indicate that the GNSs-rich tribofilms are smeared on the wear scar along the sliding direction. GNSs are self-lubricating for their ease of interlayer sliding and formation of rollers during sliding wear, which enable the reduction in friction. Graphical abstract Unlabelled Image Highlights • The incorporation of graphene in the aluminum composite contributed to reduction in wear and friction. • The dispersion of graphene in the tribolayer and the interfaces was observed. • Graphene strengthens the formation of a protective tribolayer by wrapping and bridging mechanisms. • Graphene-rich tribofilms are smeared on the wear scar along the sliding direction and suppress the friction. [ABSTRACT FROM AUTHOR]

The low coefficient of friction (COF) of sp 2 rich non-hydrogenated diamond-like carbon (a-C) coatings make them suitable for tribological applications including machining of lightweight alloy castings and for certain components in internal combustion engines. Retaining the low COF of a-C at temperatures >100 °C is however a challenge that has limited further industrial applications of these coatings. This study examines the high temperature stability of a non-hydrogenated a-C coating (H < 2 at.%) sliding against a sp 3 rich fluorinated tetrahedral amorphous carbon (ta-C-F) and tool steel counterfaces using ball-on-disk type experiments. It was shown that a-C coated balls run against ta-C-F maintained a low steady state COF that varied between 0.1 and 0.2 up to 300 °C while a-C sliding against uncoated tool steel resulted in high COF values accompanied with high wear rates of a-C coating. Transfer layers were formed on the a-C coatings’ contact surfaces sliding against ta-C-F and these layers consisting of fluorine incorporating carbonaceous material remained stable up to 300 °C. The results suggested that a low COF in this tribological system can be achieved at elevated temperatures as a result of passivation of the surface carbon atoms. [ABSTRACT FROM AUTHOR]

Recently, we presented the tribological evaluation of self-lubricating sintered steels produced by taking advantage of the powder injection molding process, the recently introduced plasma-assisted debinding and sintering process, and the in situ formation of solid lubricant particles. This new processing route promotes the in situ generation of nanostructured turbostratic graphite particles during silicon carbide dissociation. In this work, we present the influence of surface finishing on the tribological behavior of self-lubricating composites sintered at 1150°C with (3 and 5 wt%) and without SiC additions. We discuss the effects of the surface topography (Ra) on the friction coefficient and wear rates of specimens and counterbodies. The tribological behavior was analyzed using linear reciprocating sliding tests (constant load of 7 N, 60-min duration). It was shown that the reduction in surface roughness increased both the friction coefficients and wear rates of specimens and counterbodies, probably due to plastic deformation and consequent graphite reservoir sealing. Chemical analyses of the wear scars using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis showed a tribolayer that was composed predominantly of carbon and oxygen. Analyses of the wear scars showed traces of plastic deformation on both samples and counterbodies and the predominance of abrasion as the main wear mechanism. [ABSTRACT FROM AUTHOR]

As the number of annual Total Hip Replacement (THR) surgeries continues to increase, the longevity of metal based hip implants is a major concern. Tribochemical reactions cause the Cobalt-Chromium-Molybdenum (CoCrMo) hip implant to release wear debris that interacts with decomposed proteins to form a tribolayer. We conducted an electrochemical investigation in order to understand the role of molybdenum in the stability of the tribolayer under mechanical wear and electrochemical corrosion. Tribolayers made of a bovine calf serum (BCS) and bovine calf serum with sodium molybdates (BCS-Mo) were electrochemically deposited on high carbon CoCrMo discs and subjected to corrosion and tribocorrosion experiments under potentiodynamic conditions in a hip-simulator. Potentiodynamic-polarization curve, Electrochemical Impedance Spectroscopy (EIS) tests and surface characterization techniques were carried out. The results indicate an increased resistance to corrosion under mechanical wear by BCS-Mo coated surface which is more distinct when Mo is added into the electrolyte. In addition, biocompatibility evaluation using MG63 osteosarcoma cells on BCS and BCS-Mo coated samples did not show any statistically significant difference in cell growth compared to uncoated CoCrMo discs. These findings suggest a pre-formed electrochemical tribolayer with sodium molybdates may be a promising pre-implantation treatment of THRs to extend the longevity of implants in vivo. [ABSTRACT FROM AUTHOR]

DLC coatings and nanostructured carbon coating have been successfully used to prevent against wear and corrosion. Their thermal stability and internal stress have been improved by the addition of transition metals. This work characterizes the surface morphology against two different materials and growth mechanisms of an hydrogen-free carbon coating doped with a W–Mo. The wear resistance is evaluated under dry and room temperature by a set of pin on disc tests at different load and against two different counterfaces, Al 2 O 3 and stainless steel 440 C. The as-deposited and worn surfaces were characterized by electron microscopy techniques, interferometry, nanoindentation and Raman spectroscopy. The as-deposited coating presented a hardness of 14 GPa and an elastic modulus of 179 GPa with a dense surface finished and a columnar structure. The average friction coefficient was between 0.15 and 0.25, with almost no wear on the counterfaces. The W–Mo doped DLC coating showed high resistance against wear with wear rates between 3.79×10 −8 mm 3 N −1 m −1 and 2.65×10 −7 mm 3 N −1 m −1 due to its Mo–W carbide content in the amorphous matrix. A major presence of carbides prevent from adhesion to the counterface by reducing the number of dangling bonds. [ABSTRACT FROM AUTHOR]

High density TiC-based cermets were fabricated using a simple melt-infiltration/sintering method, with three different grades of stainless steel binder. Reciprocating wear tests were conducted using a ball-on-flat geometry, with the test material sliding against a WC-Co counter face. The morphology of the worn surface and wear debris were investigated using SEM and EDS chemical analysis. FIB microscopy was applied to study sub-surface damage within the wear tracks, to aid explanation of the operative wear mechanism(s). The wear behaviour is moderately complex, transitioning from two- to three-body abrasive wear, with both steel binder extrusion and TiC fragmentation observed to contribute towards third-body formation. A further transition to adhesive wear is also observed, with tribolayer generation on both tribo pair surfaces. [ABSTRACT FROM AUTHOR]

Parasitic friction losses during piston ring-cylinder liner interactions in an internal combustion (IC) engine consume large portions of the available fuel energy. This work investigates whether the tribological performance of grey cast iron (CI) cylinder liner would be improved using a piston ring coated with non-hydrogenated diamond-like carbon (NH-DLC) in comparison to uncoated CI and steel rings. Coefficient of friction (COF) values and volumetric wear losses of CI were determined using ball-on-disk type tests as a function of sliding distance. CI tested against itself during boundary lubricated sliding with synthetic engine oil showed a COF of 0.14 and a volumetric wear of 11.0×10 −4 mm 3 after 6×10 5 cycles, whereas the use of NH-DLC coated counterface resulted in a COF of 0.11 and lesser wear of 0.5×10 −4 mm 3 . Dilution of the engine oil by ethanol containing E85 biofuel was beneficial as COF was further reduced to 0.08 for CI tested against NH-DLC while maintaining low wear of 0.2×10 −4 mm 3 . According to TEM and XPS analyses, an oil residue layer (ORL) formed on the CI contact surfaces as a result of sliding-induced degradation of zinc-dialkyldithiophospahte (ZDDP) additive in the oil. The ORL, which consisted of nano-crystalline particles of sulphides and phosphates of zinc (anti-wear components) embedded in an amorphous carbon matrix, was responsible for maintaining low wear. Ethanol dilution of the synthetic oil facilitated the formation of ORL. TEM/EELS studies of the NH-DLC counterface provided evidence for OH adsorption and passivation of carbon bonds at the surface that reduced the friction. It is anticipated that piston rings coated with NH-DLC run against CI liners would show low friction and wear during boundary lubricated sliding, an effect that could be enhanced when ethanol diluted oil is used. [ABSTRACT FROM AUTHOR]

Purpose This paper aims to study microstructure and high temperature tribological performance of hypo-eutectic CoCrWC hardfacing alloy (Stellite 12) deposited on steel substrate by plasma-transferred arc (PTA) welding technique.Design/methodology/approach Microstructural characterization of the deposited coating was made using electron probe microanalysis, X-ray diffraction and microhardness tester. Dry sliding wear tests were carried out with a ball-on-disc type tribometer at room and elevated temperature. Worn surfaces of the samples were examined by the EDX equipped SEM and Raman spectroscopy.Findings Results revealed that at room temperature and 300°C plasticity dominated wear mechanism was operative. Under oxidation dominated wear conditions (400, 500, 600 and 700°C), testing temperature plays a crucial role on the characteristics of the oxide tribolayers formed on worn surfaces. Development of Cr2O3 in the tribolayer at 600 and 700°C was beneficial in increasing wear resistance of examined coating.Originality/value While the sliding wear performance of Stellite alloys at room temperature has been investigated in details, published studies on tribological behavior of Stellite alloys with varying temperature are scarce. Therefore, the present work was undertaken to study the wear mechanisms and the type of tribolayers formed during sliding wear of PTA welding deposited hypo-eutectic Stellite 12 coating with increasing temperature up to 700°C. [ABSTRACT FROM AUTHOR]

The search for Mo-Si-B based alloys exhibit excellent high-temperature wear resistance is highly demanding for their structural applications. For this target, the Mo-Si-B-xTi (x = 10, 20 and 30 wt%) alloys were fabricated and their dry sliding tribological properties at 25–1000 °C in air were systemically investigated. It indicates that the tribological properties of the MoSiBTi alloys are greatly determined by the test temperature and Ti content that correlates to oxidization process and oxidation products. Compared with other alloys, the Ti30 alloy shows an excellent wear resistance at 600–1000 °C due to the formation of dense tribo-oxides layer and improved oxidation resistance. Meantime, the friction coefficient at 800 °C is as low as 0.31 because of the formed self-lubricating tribolayer. • Dry sliding tribological properties at 25–1000 °C were systemically investigated. • Tribological behavior is greatly influenced by Ti content and test temperature. • Oxidization process and products determines the lubricating and wear mechanism. • COF of 0.26–0.31 can be obtained at 800 °C because of the excellent self-lubricity. • Ti30 alloy with 30 wt% Ti shows an excellent wear resistance at 600–1000 °C. [ABSTRACT FROM AUTHOR]

Protective coatings for high temperature applications have to exhibit good oxidation resistance and microstructure stability. The effective protection of parts with hardfacing techniques depends on the impact of the substrate on the deposited alloy, typically assessed by dilution. This investigation aims to evaluate the effect of dilution on microstructure and high temperature wear of NiCrSiBC coatings processed by plasma transferred arc on a stainless steel substrate. The microstructure was characterized by EDX, XRD, SEM and hardness. High temperature wear behavior was assessed by ball on disc tests with an alumina sphere sliding against the NiCrSiBC coating. Results showed that an increase on dilution suppressed the formation of Cr 5 B 3 borides reducing hardness. Wear behavior was ruled by the combined effect of dilution and test temperature. The effect of dilution is two fold, altering the microstructure and oxidation resistance. At low wear temperatures, the microstructure rules the performance and harder coatings exhibited a better resistance. At high temperature, wear depends on the tribolayer formed. The low oxidation resistant Fe rich coatings form more debris during sliding. The more compact tribolayer formed under these conditions covers most of the area of the wear track and accounts for the enhanced performance of high dilution coatings. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO) nanosheet coatings are considered as promising solid lubricants. They, however, still suffer short lifetime and low load capability, limiting their further mechanical engineering industrial application. In this work, copper ions are in-situ incorporated into GO nanosheet coatings by applying a low potential towards a copper substrate in a GO suspension. The generated copper ions not only induce chemical cross-linking of neighboring GO nanosheets via bounding to carboxylic acid groups at the edges of individual GO nanosheets but also reduces the C O functional groups on their basal planes. These factors facilitate retaining GO nanosheets with few structural deteriorations in the contact interface when copper covered with these coatings slides against steel ball. Consequently, the COF is as low as 0.17 even under a maximum Hertz contact pressure of 1.1 GPa, the durability is 10 times higher than the air-sprayed GO coating and the wear rate of the copper substrate is reduced more than 20 times compared with the bare copper substrate. [Display omitted] • A low-cost and green method is reported to deposit graphene oxide (GO) coatings. • GO coatings show low friction coefficient, long durability and high-load capability. • Mechanism of improved tribological performance is explored via various techniques. [ABSTRACT FROM AUTHOR]

In this study, a high quality chromized coating was produced on GCr15 steel substrate by vapor phase chromizing technique for sprag clutch application. The microstructure characteristics, mechanical properties and sliding friction behaviour were systematically investigated. The results show that the chromized coating consists of 1) an outmost layer of columnar Cr 7 C 3 grains with excess carbon atoms (55 at.%) and considerable amount of iron atoms (6.2 at.%); and 2) an inner layer of coarse Fe 3 C grains. The carbide phase formation can be well explained via the thermodynamics evaluation for the Fe-Cr-C system, while the origin of carbon contamination is likely originated from the side reactions of precursor gases. The high surface hardness (16.6 GPa) and interfacial adhesion (~50 N) endow the chromized coatings with superior wear resistance against steel counterface under ambient conditions. The wear rates are found to decrease with the applied load (10–25 N). Scanning electron microscopy (SEM) analysis of the worn surfaces indicates that a chromium oxide tribolayer tends to be formed under a higher loading which displays lubrication and hence protects the contacting material during the friction process. • A novel Cr 7 C 3 /Fe 3 C bilayer coating has been produced on GCr15 bearing steel by gas phase chromizing technique. • The chromized coating displays high surface hardness and good interfacial adhesion. • During sliding friction, the formation of chromium oxide tribolayer helps lower the wear rates effectively. [ABSTRACT FROM AUTHOR]

Abstract: TiC-based ceramic–metal composites, or cermets, are widely used in applications requiring wear and corrosion resistance. In the present work, a family of novel TiC–stainless steel (grade 304L) cermets has been developed using vacuum melt infiltration (1500°C/1h), with steel binder contents varied from 5 to 30vol%. Microstructural analysis showed a homogenous distribution of TiC within the steel binder, with mean TiC grain sizes of ∼6μm. Increasing the steel content resulted in an increase in the indentation fracture resistance and a decrease in the hardness. The reciprocating wear resistance of the cermets was assessed using a ball-on-flat geometry, using a WC–Co sphere dry sliding on the polished cermet surface. It was shown that there is an increase in the specific wear rate with both increasing load and binder content. Similarly, a higher coefficient of friction was observed with higher steel binder contents. The morphology of the worn surface was investigated using scanning electron microscopy, and associated energy dispersive X-ray spectroscopy, to more fully understand the operative wear mechanisms. Evidence of a transition from two- to three-body abrasive wear was observed, together with the formation of a tribolayer, indicating that adhesive wear was also occurring. [Copyright &y& Elsevier]

Abstract: Friction and wear behaviors of W–C:H coatings with different tungsten contents sliding against bearing steel balls at different air humidities were investigated. The worn out surfaces of steel balls and coatings were analyzed with the aid of scanning electron microscopy (SEM) and Raman spectroscopy. A tribolayer composed of a graphite-like material mixed with tungsten and iron oxides was observed on the friction surfaces of the steel balls. The chemical and phase compositions of the tribolayer, which depend both on the tungsten content in coatings and air humidity, determine the tribological properties of the W–C:H coating in a frictional contact with bearing steel. At average air humidity (50%), those coatings that contain less than 10at% of tungsten in a frictional contact with steel exhibit favorable tribological properties. The friction coefficient of frictional contacts under test reaches a low value (f∼0.01) at a low air humidity and increases with humidity of up to ca. 0.2. The best tribological properties in a wide range of air humidity (5–90%) have been found for W–C:H coatings with the tungsten content between 2 and 5at%. [Copyright &y& Elsevier]

Abstract: Composite coatings consisting of an Al–12Si alloy matrix reinforced with TiB2 particles were prepared by laser cladding, from mixtures of Al–12Si alloy and TiB2 powders. The coating''s microstructure is composed of TiB2 particles dispersed in a matrix consisting of proeutectic α-Al dendrites and α-Al+Si eutectic. The coatings present a hardness of 156 HV and a wear coefficient of 2.65×10−5 mm3/(Nm) in dry sliding wear tests, with a counterbody of AISI 440C tool steel. The interaction between the sample and the counterbody leads to the formation of a tribolayer containing of a large proportion of oxides of the elements both of the sample and of the counterbody. The formation of this tribolayer can be explained by the oxidation of the tribological pair materials due to the high flash temperatures generated during asperity interactions, followed by cracking of the oxides and consolidation of the debris into a compact surface layer which protects the coating from further wear. Repeated loading causes cracking of the tribolayer and its delamination as the main material removal mechanism. [Copyright &y& Elsevier]

Abstract: This paper illustrates the distinctive capabilities of using focused ion beam (FIB) techniques to elucidate the microstructure that forms beneath the worn regions of samples subjected to laboratory tests under ultra-mild wear (UMW) conditions and those extracted from specific regions of a worn surface of engine blocks. TEM investigations of cross-sectional samples excised with a focused gallium-ion beam to obtain electron transparency of the first few micrometers of the contact surface were effective in providing novel information on the micromechanisms of UMW that are not readily obtained with conventional microscopy. Two FIB sample preparation techniques, namely the lift-out and H-bar techniques were used to prepare site-specific cross-sectional TEM specimens from Al–11% Si alloys subjected to pin-on-disk and dynamometer tests. In both cases, the aluminum matrix was heavily deformed in the top few micrometers below the contact surface, which resulted in the formation of ultrafine grains. Damage to the silicon particles was also observed. Additionally, a tribolayer (or an oil-residue layer) whose thickness increased with sliding cycles, was developed on the contact surfaces. It was concluded that the development of the oil residue layer supported by a layer of ultrafine grain structure was necessary to maintain the low wear rates. [ABSTRACT FROM AUTHOR]

Abstract: Friction and wear properties of nanostructured anodic aluminum oxide (AAO)) films were studied in relation to contact load and pore size (pore diameter). Uniformly arrayed nanoporous aluminum oxide films (pores of 28nm, 45nm, 95nm, and 200nm diameter and 60–100μm thick) were synthesized by anodization. Reciprocating wear tests using 1mm diameter steel balls as counterpart were carried out for a wide range of load (from 1mN to 1N) at ambient environment. The friction coefficient reduced with the increase of load. The friction coefficient decreased by approximately 30% when the load increased by 3 orders of magnitude. The pore density marginally affected the frictional properties of AAO films. The influence of pore size on the friction coefficient was significant at relatively high loads (0.1N and 1N) whereas it was negligible at low loads (1mN and 10mN). The worn surface of AAO films tested at low loads did not experience tribochemical reaction and exhibited only mild plastic deformation. Dispersed thick smooth films were formed on the worn surface of all samples at relatively high loads whereas only extremely thin smooth film patches were rarely formed at low loads. These thick smooth films were generated by combined influence of tribochemical reaction at the contact interface and plastic deformation of compacted debris particles as evidenced by energy-dispersive spectroscopy analysis. We suggest that these thick films mainly contributed to the decrease of friction regardless of the pore size. [ABSTRACT FROM AUTHOR]

Abstract: This study employed thermal oxidation (TO) treatment as a way of improving the wear resistance of Ti–6Al–4V/10vol.% TiC composites (TMCs) – especially at high loading conditions. The study revealed the formation of a uniform oxide layer (2.6±0.35μm thick) supported by an oxygen diffused zone (31.2±11.6μm deep) on the composite''s surface when thermal oxidation was carried out at 800°C for 20min. The dry sliding wear behaviour of the TO-treated and untreated TMCs was investigated using a ball-on-disk tribometer within a load range of 2–10N. According to the experimental results, different wear behaviours were identified for TMCs and TO-TMCs as a function of applied load. The TO-treatment significantly reduced the wear rates of TMCs, especially at high loading conditions, due to the presence of a hard oxide layer and diffused zone that not only enhanced the load-bearing capacity of the TMCs, but also hindered the abrasive effect of the TiC particles and the extensive plastic deformation of Ti matrix. [Copyright &y& Elsevier]

Abstract: This paper presents the results of sliding wear experiments conducted on high-purity titanium (Ti) against bearing-steel in liquid nitrogen (LN2; boiling point: 77K) environment. Ti samples of three different grain sizes (9, 17 and 37μm) were used to study the effect of hardness, derived from grain refinement as well as cryogenic test temperature, on the wear properties of Ti. In our experiments, a constant load of 10N and sliding speeds of 0.67, 1.11 and 4.19ms−1 were used. The coefficient of friction (COF) for this tribo-couple varied between ∼0.25 and ∼0.50. While a steady state was always achieved, a peak in the COF was always noted in case of coarse-grain (37μm) Ti tested at a sliding speed of 4.19ms−1. Under the investigated sliding conditions, the wear rate was found to be of the order of 10−3–10−4 mm3 N−1 m−1. The lowest wear rate was recorded in the fine-grain (9μm) Ti at the highest sliding speed of 4.19m s−1. The critical analysis of the worn surface topography reveals that the reduced wear rate was due to the formation of adherent and strain-hardened tribolayer. In order to show various dominant wear mechanisms of Ti, a qualitative map was developed in sliding speed–grain size space. Substructure evaluation revealed the formation of a dense array of deformation twins because of the plastic deformation, which often resulted in the subdivision of grains. [Copyright &y& Elsevier]

In this study, dry sliding wear behavior of Al alloy (Al 2219) based metal matrix friction composites (AlMMFCs) incorporated with varying percentage of ingredients: silicon carbide particles (15-25 wt % SiCp) and solid lubricants with 4 wt % graphite and 1 wt % antimony trisulphide (Sb2S3) were investigated. A group of four new chemical formulations, three binary composites of Al/SiCp (Al01N, Al02N, and Al03N), and a hybrid composite of Al/ SiCp/ solid lubricants (AIO4N) were fabricated by newly a developed "cold-hot powder die compaction" method. Physical and mechanical properties were measured as usual, To measure tribological properties, dry pin-on-disk wear tests were conducted for 1 hour at varying loads of 1 MPa and 2 MPa and at sliding speeds of 3 m/s, 5 m/s, 7 m/s, and 9 m/s. The results revealed that the incorporation of SiCp from 15 wt % to 25 wt % in binary composite, density (2.8-2.9 g/cc), apparent porosity (1.4-3.4 vol %), and hardness (78-93 BHN) were increased. For hybrid composite, density (2.9-2.76 g/cc) and hardness (93-81 BHN) were decreased with the increase in apparent porosity (3.4-4.1 vol %). It was concluded that the obtained density is higher than the reported density and the obtained apparent porosity is much lower than the reported apparent porosity by Aigbodi et al. (2007, "Effects of Silicon Carbide Reinforcement on Microstructure and Properties of Cast Al-Si-Fe/Sic Particulate Composites," Mater. Sci. Eng., A, 447, pp. 355-360) for same composition using "double stir casting" method. The value of coefficient of friction with addition of solid lubricants increased and steady at high load and speed (2 MPa, >5 m/s).The microstructures, worn surfaces, and tribolayers are also analyzed by an optical microscope and SEM. This study overviews AlMMFCs incorporated with hard particles and solid lubricants and the new technology for producing brake lining parts from these novel materials. [ABSTRACT FROM AUTHOR]

Abstract: The wear behavior of polytetrafluoroethylene (PTFE) and polyimide (PI) has been comparatively evaluated under dry sliding and simulated sand-dust conditions. An improved block-on-ring wear tester equipped with an attachment for simulating the sand-dust environment was used to evaluate the abrasive wear behavior of materials. The sand collected from the Yellow River of China was used to simulate the sand-dust environment, also different loads and sand-dust sizes were chosen for tribological tests. The two chosen polymers showed different wear behavior under sand-dust conditions and the wear rates of PTFE were much lower under sand-dust conditions than under dry sliding conditions. This was attributed to the formation of the tribolayer on the worn surfaces during the abrasive wear process. The sand-dust enhanced the wear resistance of PTFE, but reduced that of PI because, in contrast to PTFE, there was no tribolayer formed on the PI worn surface. The wear rate of PTFE increased under sand-dust conditions while the wear rate of PI decreased with the increase of applied load. The higher hardness of PI and fragmentation of abrasive particles under high loads accounted for the decrease in wear rate as load increased. [Copyright &y& Elsevier]

• A novel reinforcement made by chips and powder with same composition was used. • Composite fabrication was done by casting method in semi-solid stage. • Tribo-layer generation was identified during the dry sliding wear test. • Worn surface topography was analyzed by the profilometry, XRD, FESEM and EDS. • ANN model with two stage nested theory is applied for the experimental results. The influence of powder-chip based reinforced LM6 aluminum alloy fabricated by a consolidated effect of stirring and squeeze process in the semi-solid stage is reported for wear properties. Effect of oxide formation on the worn surfaces due to the processing was noticed and experimental results showing that powder-chip based reinforcement with semi-solid slurry affects and gave excellent resistance against the adhesive wear. Evidences of protective tribo-layers observed from the worn surface investigation, profilometer analysis, EDS and XRD results which provides an appropriate explanation for the drop in the wear rate in alloys. Specific wear rate reduction due to the effect of oxides in mixed tribolayer has been studied by artificial neural network (ANN) with two stage nested analysis which reflects only 1.11% Mean Square Error as compared to experimental values. This model provides better understanding to identify influencing parameter for huge variable set of processing and validate with sufficient accuracy. [ABSTRACT FROM AUTHOR]

Cu-based brake pads for high-speed train were tested in full-scale at braking speeds as high as 380 km/h. The microstructure, composition and properties of the original, worn and lateral surfaces were investigated. The results show that a ~2 μm thick nanostructured tribolayer composed of CuO and Fe 2 O 3 was formed on the worn surface, owing to the plastic deformation, mechanical mixing, oxidation and sintering processes. The tribolayer enables significantly higher friction coefficient and wear resistance than original surface, which can be attributed to oxide-based composition and high hardness. The graphite particles in the worn and lateral surfaces were burned and the copper matrix was oxidized. The tribological property of brake pad highly depends on the composition and microstructure of the tribolayer. Image 1 • Cu-based brake pads for high-speed train were tested in full-scale braking at 380 km/h. • A nanostructured tribolayer composed of CuO and Fe 2 O 3 was formed on the worn surface. • The tribolayer enables high friction coefficient and excellent wear resistance. • Oxidation, delamination and spallation are the dominant wear mechanisms. [ABSTRACT FROM AUTHOR]